Sudden cardiac death (“SCD”) can be generally defined as death within one hour of the onset of symptoms without a previously-known disease or a disease that was expected to be lethal. SCD can also occur without symptoms or warnings signs. SCD is often described with respect to an unwitnessed death with the victim having been known to be alive less than 24 hours earlier.
SCD can kill its victims within minutes and often occurs in outwardly healthy people who have no known heart disease. Although it may occur in outwardly healthy people, most victims do have heart disease or other health problems, often without being aware of it. SCD claims about 300,000 lives a year in the United States and presents a public health challenge in that often the only indication a patient is at risk appears when the patient succumbs, without warning, to a heart failure episode.
In many cases, SCD victims suffer from ventricular tachycardia that degenerates into ventricular fibrillation. Ventricular tachycardia is a type of cardiac arrhythmia that is a serious, often-times, fatal condition characterized by rapid, uncontrolled, and ineffective beating of the heart. Ventricular fibrillation is a chaotic ventricular heart rhythm which produces little or no net blood flow from the heart, such that there is little or no net blood flow to the brain and other organs. Ventricular fibrillation, if not terminated, results in death. Researchers continue their efforts to predict the onset and triggers for such ventricular tachyarrhythmias and SCD.
Sleep apnea is a condition that can cause serious medical problems if left untreated. The risk of heart disease and stroke increases, and the probability of having traffic accidents also increases. Patients most likely to develop sleep apnea include those who snore loudly, are overweight, have high blood pressure, or have some physical abnormality in the nose, throat, or other parts of the upper airway.
Sleep apnea is a breathing disorder characterized by brief interruptions of breathing during sleep. There are two types of sleep apnea: central and obstructive. Central sleep apnea occurs when the brain fails to send the appropriate signals to the breathing muscles to initiate respiration. Obstructive sleep apnea occurs when air cannot flow into or out of the patient's nose or mouth although efforts to breathe continue.
Sleep apnea is thought to be caused by certain mechanical and structural problems in the airway which cause the interruptions in breathing during sleep. In some patients, sleep apnea occurs when the throat muscles and tongue relax during sleep and partially block the opening of the airway, making breathing labored and noisy, and even stopping breathing altogether. With a narrowed airway, the patient continues his efforts to breathe, but air cannot easily flow into or out of the nose or mouth. Unknown to the patient, this results in heavy snoring, periods of no breathing, and frequent arousals, which are abrupt changes from deep sleep to light sleep. During an apneic event, the patient is unable to breathe in oxygen and to exhale carbon dioxide, resulting in low levels of oxygen and increased levels of carbon dioxide in the blood. The reduction in oxygen and increase in carbon dioxide alert the brain to resume breathing and cause an arousal. With each arousal, a signal is sent from the brain to the upper airway muscles to open the airway, and breathing is resumed, but often with a loud snort or gasp.
In normal sleep, the body transitions from wakefulness to non-rapid eye movement. During this phase, there is a reduction in central respiratory drive. This results in a regular pattern of breathing. At approximately the same time, the patient's heart rate, blood pressure, stroke volume, cardiac output, and systemic vascular resistance all decrease. This results in a state of hemodynamic and autonomic relaxation during which myocardial workload is reduced. A patient with sleep apnea does not experience a regular pattern of breathing, and as a result, the body has different hemodynamic and cardiovascular responses.